latest in the george zimmerman murder trial. An emotional and very difficult day for the prosecution's star witness, who was talking with trayvon martin just before he was shot. She got a grilling from the defense. And abc's matt gutman is in sanford, florida, with a look at it all. Good morning, matt. Reporter: Good morning, josh. Analysts tell us that rachel jeantel's testimony is the biggest thing standing between george zimmerman and an acquittal. That's one reason his defense team worked so relentlessly to discredit her. For the second-straight day, george zimmerman's attorney kept rachel jeantel on the stand to tear her down. You didn't do anything because trayvon martin started the fight and you knew that. Objection. Compound question. Badgering the witness. Reporter: Jeantel was on the phone with trayvon martins in the seconds before neighborhood watchman, george zimmerman, shot him. Martin told her that he was being followed. And she heard zimmerman confront him. During a heated cross-examination, lasting more than five hours. Are you claiming in any way that you don't understand english? I don't understand you. Reporter: Zimmerman's legal team tried to hack away at her ce credibility. You said you could have heard trayvon martin smashing george zimmerman in the face. By who? By you. Reporter: And in a jarring exchange, don west accused the 19 about lying about a letter she wrote to martin's mother, explaining what she saw that night. You didn't tell the prosecutor about it? No. Reporter: Asked to read a letter she asked a friend to help her write, she could not. Are you able to read that at all? Some of it. I do not. Can you read any of the words on it? I don't understand, um, cursive. I don't read cursive. Reporter: Later this morning, abc news has learned jurors are expected to hear from this man, chris serino, the lead homicide investigator osen the case. In the days after the shooting, zimmerman who pleaded not guilty, recorded this re-enactment, showing serino, how he says martin attacked him. Slamming his head into the pavement. Saying he only shot in self-defense. Now, sarino was skeptical, that zimmerman had to shoot martin because of a life or death struggle. That's why he suggested manslaughter charges against zimmerman. He could take the stand later today. We want to turn to abc's chief legal affairs anchor, dan abrams. And rachel jeantel, more composed than the first day on the witness stand. But seven, long hours being grilled. What did it do to the prosecution? She's a flawed witness. This is not the witness you want if you're the prosecution. And this is the last person who spoke to trayvon martin. There's inconsistencies in her story. As you heard, she was attacked, again and again on cross-examination. With that said, you saw that section there where she talks about how she can't read cursive. She doesn't seem educated. But don't underestimate her. She's still smart. The attorney was trying to ask her questions to get her to catch things. And she said, that's not what i said. While she's not worldly, so to speak. She understand what she was being asked. She understood the questions. And she took on the defense attorney, again and again, as he was trying to question her. We call rachel jeantel the most important witness for the prosecution's case. But there was more testimony yesterday that may help buttress the idea that george zimmerman was the aggressor hoop. That's the big question. Another witness said she believes that george zimmerman was on top when they were fighting. That's one of the key questions. Everyone agrees there was a confrontation. They don't agree when did it start, where did it start and who was on top? And the defense's position is that it was trayvon martin who attacked george zimmerman. If the prosecution can show zimmerman was on top, it's an important point.

This transcript has been automatically generated and may not be 100% accurate.

{"id":19517867,"title":"Trayvon Martin Friend Grilled by Defense","duration":"4:33","description":"Rachel Jeantel, who spoke to Martin just before he was shot, was on the stand for seven hours.","url":"/GMA/video/george-zimmerman-trial-trayvon-martin-death-friend-rachel-19517867","section":"GMA","mediaType":"Default"}